DS-CDMA or CDMA communication systems are being implemented as cellular telephone systems. A CDMA system includes a system controller and at least one base station. Each base station provides communication service to a fixed geographic area or cell. Mobile stations in a cell communicate with the base station for that cell. Communication with a mobile station is handed off among base stations as the mobile station moves among cells. One example of such a system is a system according to a EIA/TIA interim standard 95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System ("IS-95").
A typical DS-CDMA transmitter 100 is shown in FIG. 1. The transmitter accepts information bits. These may be digitized, compressed voice or digital data formatted in an appropriate protocol. These bits are encoded for error correction and interleaved in encoder and interleaver 101. The resulting binary data stream is mapped from binary data (0,1) to symbols (-1,+1) in the binary-to-numeric block 102. Each symbol of the emerging data stream is multiplied by a length N Walsh code, where N is normally an integer power of 2, by multiplier 103, in a process referred to as Walsh covering. The duration of each element of the Walsh code is referred to as the chip duration and the inverse of this quantity is the chip rate. Because all length N Walsh codes are orthogonal to each other, this will allow the receiver to separate individual uses by correlation to a received signal with the given Walsh code. The sequence formed by the Walsh covering is then multiplied by a complex spreading sequence. This is accomplished by performing two real multiplies, one in which the Walsh covered sequence is multiplied by a first psuedorandom noise sequence PNi by multiplier 104 to form the in-phase channel and the other by a second sequence PNq in multiplier 105 to form the quadrature component of the complex baseband signal. Note that the spreading sequence formed by PNi and occupy a quartenary phase shift keyed (QPSK) constellation, and therefore will be referred to as a QPSK modulation or a QPSK spreading sequence. In general, a spreading sequence will be considered any sequence with relatively uniform spectrum over a desired range which is multiplied by a second sequence for the purpose uniformly distributing a signal across the extent of the desired band. For systems such as IS-95, in which offset quartenary phase shift keying is specified for subscriber unit transmission, the quadrature component of the complex baseband signal is delayed by 1/2 chip by delay element 106. Both the delayed quadrature and the in-phase component of the signal are then filtered by identical spectral shaping filters 107 and 108 to prevent out-of-band emissions. The filtered in-phase component is then multiplied by cos(.omega.t) in multiplier 113 and the filtered quadrature component by sin(.omega.t) in multiplier 109 and the resulting signals summed in summer 110 to up-convert the baseband signal to the desired carrier frequency. The modulated carrier produced by summer 110 is then amplified by power amplifier 111 to the desired power level.
When the in-phase and quadrature signals are scaled in filters 107 and 108, the peak magnitude of the output of the in-phase and/or quadrature signals will exceed the average output magnitude. The ratio of the peak magnitude of the filter output to the average level is referred to as peak-to-average ratio. High peak-to-average ratios are undesirable because the power amplifier 111 must be linear over the entire signal range, including the peak value. Therefore the peak signal level determines both the size and bias requirements of the power amplifier. High peak-to-average ratios therefore imply higher current drain, large size, and more costly power amplifiers. These characteristics become very important in low cost, battery powered subscriber units.
Attempts have been made to reduce the peak-to-average ratio to eliminate the necessity of increasing the capacity of the power amplifier. Focus has been placed mainly upon the signal spreading scheme since certain schemes have been found to directly reduce the peak-to-average ratio. These sequences must be chosen in a mariner which not only reduces the peak-to-average ratio but also has a short duration auto-correlation to preserve the interference averaging properties of QPSK spreading.
Accordingly, a need exists for a modulation scheme which minimizes linearity requirements in single code scenarios.